Question

Use De Moivre's Formula to derive the identity sin(40) = 4 cos (0) sin(e) - 4 cos(e) sin (0) as well as a similar "quadruple angle" formula for cos(40). S eeeotoble to leave answer in polar or

Answer 1

By DeMoivre's theorem,

`\cos(4\theta)+i\sin(4\theta)=(\cos\theta+i\sin\theta)^4`

Expanding the right side gives

`\cos^4\theta+4i\cos^3\theta\sin\theta-6\cos^2\theta\sin^2\theta-4i\cos\theta\sin^3\theta+\sin^4\theta`

Equating imaginary parts tells us

`\sin(4\theta)=4\cos^3\theta\sin\theta-4\cos\theta\sin^3\theta`

(Not sure what you mean by sin(e) and cos(e)...)